Combustion Chamber Deposit Flaking Studies Using a Road Test Procedure

A new field problem associated with flakes of combustion chamber deposit (CCD) getting trapped on the exhaust valve seat has been reported by several car manufacturers in Europe. This causes difficulties in start-up and poor driveability. A road test procedure that is reasonably quick and sensitive to fuel changes has been developed to study the deposit flaking problem. The flaking of the deposits is believed to be caused by water - either generated by combustion or existing in the ambient air as water vapour - condensing on the deposits. Water is much more effective than fuel in causing deposit flaking. A way of quantifying the deposit flaking tendency has been defined and its repeatability established based on twenty-nine tests using two different cars and different fuels and additives. There are large differences between base fuels in terms of CCD flaking. Deposit flaking tendency is reduced by running the car in a higher duty cycle or by using conventional detergent additive packages particularly at higher dose rates. The extent of such an effect of additives also depends on the base fuel. The same test methodology was also used to study two different car models by another manufacturer that differed significantly in terms of the field problems associated with deposit flaking; it was found that the test results agreed with the field experience.